Foreign Object Detection in Wireless Power Transfer by Asymmetry Detection

ABSTRACT

In one embodiment, a wireless power transfer system comprises a transmitter coil structure comprising a first transmitter coil, and a second transmitter coil coupled to the first transmitter coil in such a way that when a first current flows in the first transmitter coil in a first spatial direction the first current flows in the second transmitter coil in a second spatial direction different from the first spatial direction, a foreign object sensor coil structure comprising a first sensor coil having a central axis in common with the first transmitter coil, and a second sensor coil coupled in series to the first sensor coil, the second sensor coil having a central axis in common with the second transmitter coil, the first sensor coil coupled to the second sensor coil in such a way that when a first voltage induced in the first sensor coil has a first polarity a second voltage induced in the second sensor coil has a second polarity different from the first polarity, a voltage detector coupled to the foreign object sensor coil structure, the voltage detector configured to detect a net voltage across the foreign object sensor coil structure, and a controller configured to receive the net voltage and determine a presence or absence of a foreign object based on a magnitude of the net voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 62/492,956, entitled “Foreign Object Detection inWireless Power Transfer by Asymmetry Detection,” filed on May 2, 2017,the subject matter of which is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The invention relates generally to wireless power transfer and moreparticularly to foreign object detection in wireless power transfer byasymmetry detection.

BACKGROUND

Electronic devices typically require a connected (wired) power source tooperate, for example, battery power or a wired connection to a directcurrent (“DC”) or alternating current (“AC”) power source. Similarly,rechargeable battery-powered electronic devices are typically chargedusing a wired power-supply that connects the electronic device to a DCor AC power source. The limitation of these devices is the need todirectly connect the device to a power source using wires.

Wireless power transfer (WPT) involves the use of time-varying magneticfields to wirelessly transfer power from a source to a device. Faraday'slaw of magnetic induction provides that if a time-varying current isapplied to one coil (e.g., a transmitter coil) a voltage will be inducedin a nearby second coil (e.g., a receiver coil). The voltage induced inthe receiver coil can then be rectified and filtered to generate astable DC voltage for powering an electronic device or charging abattery. The receiver coil and associated circuitry for generating a DCvoltage can be connected to or included within the electronic deviceitself such as a smartphone or tablet.

The Wireless Power Consortium (WPC) was established in 2008 to developthe Qi inductive power standard for charging and powering electronicdevices. Powermat is another well-known standard for WPT developed bythe Power Matters Alliance (PMA). The Qi and Powermat near-fieldstandards operate in the frequency band of 100-400 kHz. The problem withnear-field WPT technology is that typically only 5 Watts of power can betransferred over the short distance of 2 to 5 millimeters between apower source and an electronic device, though there are ongoing effortsto increase the power. For example, some concurrently developingstandards achieve this by operating at much higher frequencies, such as6.78 MHz or 13.56 MHz. Though they are called magnetic resonance methodsinstead of magnetic induction, they are based on the same underlyingphysics of magnetic induction. There also have been some marketconsolidation efforts to unite into larger organizations, such as theAirFuel Alliance consisting of PMA and the Rezence standard from theAlliance For Wireless Power (A4WP), but the technical aspects haveremained largely unchanged.

Wireless power transfer transmitters commonly have a flat or concavesurface on which devices to be charged are placed. Objects other thandevices that can be wirelessly charged are sometimes placed on thecharging surface of a wireless power transmitter, whether intentionallyor inadvertently. Certain metal objects such as coins, paper clips, andkeys can develop eddy currents in response to the varying magnetic fieldproduced by the wireless power transmitter. Such “foreign objects” causelosses in the power being transferred to a wirelessly-chargeable device.Also, if left on the surface of the transmitter for a period of time,the eddy currents can cause the foreign metal object to overheat,potentially causing burns or igniting a fire.

The foreign object detection technique set forth in the Qi standardcompares the amount of power transmitted by the transmitter and theamount of power that the wireless power receiver reports back to thetransmitter. The difference between the two values is a loss number.Eddy currents that develop in a foreign object, such as a coin, on thetransmitter surface will cause the loss number to increase. If the lossnumber exceeds a predetermined threshold, an indicator such as a lightemitting diode may be triggered while the transmitter continues totransmit power. If the loss number exceeds the predetermined thresholdfor a predetermined amount of time, for example ten seconds, thetransmitter will stop transmitting power. A drawback of this method offoreign object detection relates to inaccuracy in determining the actualamount of power transmitted or received. For example, a Qi-complianttransmitter indirectly estimates the AC power transmitted by the coil bysensing the DC voltage and current input to the transmitter, which cancause over 300 mW of error at 5 W. In other words, based on the input DCvalues, when a Qi transmitter determines that it is transmitting 5 W,the actual power may be a value between 4.7 W to 5.3 W. Similarly, aQi-compliant receiver also indirectly estimates the AC power reachingits coil by sensing the output DC voltage and current, which can causean error of 300 mW at 5 W. These errors at the transmitter and receivercan compound such that a foreign object escapes detection. For example,a Qi transmitter may be transmitting 5.3 W of power and the Qi receivermay be receiving 4.7 W of power, so the actual power loss is 600 mW,which can be caused by a coin or other foreign object on the transmitterabsorbing the 600 mW. But because of errors both the transmitter andreceiver sense that the power being transferred is 5 W, so the lossnumber is determined to be zero and the transmitter keeps operating.Absorbing 600 mW of power can raise the temperature of a coin to apotentially unsafe level. This problem becomes even more potentiallydangerous for systems operating at higher power levels such as 10 W.Assuming the same percentage of error, the transmitter and receiverwould each have a sensing error of 0.6 W, which could lead to a totalpower loss of 1.2 W going undetected. A coin on the transmitterabsorbing 1.2 W of power going undetected can rise to a temperature thatis a definite fire hazard. This inaccuracy in sensing transferred poweris a major drawback that prevents Qi-compliant systems from safelydelivering more than 5 W. Thus there is a need for an improved techniquefor detecting foreign objects on wireless power transmitters.

SUMMARY

In one embodiment, a wireless power transfer system comprises atransmitter coil structure comprising a first transmitter coil, and asecond transmitter coil coupled to the first transmitter coil in such away that when a first current flows in the first transmitter coil in afirst spatial direction the first current flows in the secondtransmitter coil in a second spatial direction different from the firstspatial direction, a foreign object sensor coil structure comprising afirst sensor coil having a central axis in common with the firsttransmitter coil, and a second sensor coil coupled in series to thefirst sensor coil, the second sensor coil having a central axis incommon with the second transmitter coil, the first sensor coil coupledto the second sensor coil in such a way that when a first voltageinduced in the first sensor coil has a first polarity a second voltageinduced in the second sensor coil has a second polarity different fromthe first polarity, a voltage detector coupled to the foreign objectsensor coil structure, the voltage detector configured to detect a netvoltage across the foreign object sensor coil structure, and acontroller configured to receive the net voltage and determine apresence or absence of a foreign object based on a magnitude of the netvoltage.

In one embodiment, the controller is configured to determine thepresence of the foreign object when the magnitude of the net voltage isgreater than a predetermined threshold and determine the absence of theforeign object when the magnitude of the net voltage is not greater thanthe predetermined threshold. In one embodiment, the wireless powertransfer system further comprises a power circuit coupled to thetransmitter coil structure, the power circuit configured to providepower to the transmitter coil structure. In one embodiment, thecontroller is further configured to cause the power circuit to haltproviding power to the transmitter coil structure upon determining thepresence of the foreign object. In one embodiment, the wireless powertransfer system further comprises an output device, and the controlleris further configured to cause the output device to generate auser-perceivable output upon determining the presence of the foreignobject. In one embodiment, the wireless power transfer system furthercomprises a transmitting surface overlying the transmitter coilstructure and the foreign object sensor coil structure. In oneembodiment, the voltage detector comprises a sense resistor coupled tothe foreign object sensor coil structure. In one embodiment, thewireless power transfer system further comprises a layer of magneticmaterial underlying the transmitter coil structure and the foreignobject sensor coil structure.

In one embodiment a method for detecting foreign objects on a wirelesspower transfer device comprises providing power to a transmitter coilstructure comprising a first transmitter coil, and a second transmittercoil coupled to the first transmitter coil in such a way that when afirst current flows in the first transmitter coil in a first spatialdirection the first current flows in the second transmitter coil in asecond spatial direction different from the first spatial direction,detecting a net voltage across a foreign object sensor coil structure,the foreign object sensor coil structure comprising a first sensor coilhaving a central axis in common with the first transmitter coil, and asecond sensor coil coupled in series to the first sensor coil, thesecond sensor coil having a central axis in common with the secondtransmitter coil, the first sensor coil coupled to the second sensorcoil in such a way that when a first voltage induced in the first sensorcoil has a first polarity a second voltage induced in the second sensorcoil has a second polarity different from the first polarity, comparingthe net voltage to a predetermined threshold, and determining presenceor absence of a foreign object based on a result of comparing the netvoltage to the predetermined threshold.

In one embodiment, determining presence of the foreign object comprisesdetermining that the net voltage is greater than the predeterminedthreshold. In one embodiment, determining absence of the foreign objectcomprises determining that the net voltage is not greater than thepredetermined threshold. In one embodiment, the method further compriseshalting providing power to the transmitter coil structure in response todetermining the presence of the foreign object. In one embodiment, themethod further comprises generating a user-perceivable output inresponse to determining the presence of the foreign object.

In one embodiment, a system for detecting foreign objects on a wirelesspower transmitter comprises a foreign object sensor coil structurecomprising a first sensor coil, and a second sensor coil coupled inseries to the first sensor coil, the first sensor coil coupled to thesecond sensor coil in such a way that when a first voltage induced inthe first sensor coil has a first polarity a second voltage induced inthe second sensor coil has a second polarity different from the firstpolarity, a voltage detector coupled to the foreign object sensor coilstructure, the voltage detector configured to detect a net voltageacross the foreign object sensor coil structure, and a controllerconfigured to receive the net voltage and determine a presence orabsence of a foreign object based on a magnitude of the net voltage. Inone embodiment, the controller is configured to determine the presenceof the foreign object when the magnitude of the net voltage is greaterthan a predetermined threshold and determine the absence of the foreignobject when the magnitude of the net voltage is not greater than thepredetermined threshold. In one embodiment, the system further comprisesan output device, and the controller is further configured to cause theoutput device to generate a user-perceivable output upon determining thepresence of the foreign object. In one embodiment, the system furthercomprises a transmitting surface overlying the foreign object sensorcoil structure. In one embodiment, the voltage detector comprises asense resistor coupled to the foreign object sensor coil structure. Inone embodiment, the system further comprises a layer of magneticmaterial underlying the foreign object sensor coil structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of one embodiment of a wireless power transmitterincluding a foreign object detection system, according to the invention.

FIG. 2 is a cross-sectional diagram of one embodiment of a wirelesstransmitter coil structure and a foreign object sensor coil structure,according to the invention.

FIG. 3 is a diagram of one embodiment of a foreign object detectionsystem, according to the invention.

FIG. 4 is a flowchart of method steps for detecting a foreign object ona transmitting surface of a wireless power transmitter, according to oneembodiment of the invention.

FIG. 5 is a flowchart of method steps for detecting a foreign object ona transmitting surface of a wireless power transmitter, according to oneembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a diagram of one embodiment of a wireless power transmitter100 including a foreign object detection system, according to theinvention. Transmitter 100 includes but is not limited to a transmittercoil structure including a first transmitter coil 122 and a secondtransmitter coil 124, a power circuit 128 coupled to the transmittercoil structure, a foreign object sensor coil structure including a firstsensor coil 132 and a second sensor coil 134, a voltage detector 142coupled to the foreign object sensor coil structure, and a controller146. In the FIG. 1 embodiment, transmitter 100 also includes asubstantially planar layer 110 of magnetic material underlying thetransmitter coil structure (first transmitter coil 122 and secondtransmitter coil 124) and the foreign object sensor coil structure(first sensor coil 132 and second sensor coil 134). Magnetic layer 110can be made of ferrite or any other magnetic material known in the art.In one embodiment, magnetic layer 110 is substantially planar. In otherembodiments, magnetic layer 110 includes one or more raised portions,for example raised portions in areas enclosed by first transmitter coil122 and second transmitter coil 124, raised portions in areas enclosedby first sensor coil 132 and second sensor coil 134, and/or a raisedportion in an area between first transmitter coil 122 and secondtransmitter coil 124. In one embodiment, first transmitter coil 122 andsecond transmitter coil 124 have the same number of turns, substantiallythe same area, and are wound in the same direction. In the FIG. 1embodiment, first transmitter coil 122 and second transmitter coil 124are both wound in the clockwise direction. First transmitter coil 122 iscoupled to second transmitter coil 124 through a pathway 126 in such away that when a current is flowing in first transmitter coil 122 in afirst spatial direction (e.g., counter-clockwise), a current is flowingin second transmitter coil 124 in a second spatial direction (e.g.,clockwise). In one embodiment, first sensor coil 132 and second sensorcoil 134 have the same number of turns, substantially the same area, andare wound in the same direction. In the FIG. 1 embodiment, first sensorcoil 132 and second sensor coil 134 are both wound in the clockwisedirection. First transmitter coil 122, second transmitter coil 124,first sensor coil 132, and second sensor coil 134 can be formed of wireor traces on a printed circuit board using conductive material such ascopper, gold, or any other conductive material known in the art.

Power circuit 128 generates an AC signal that provides power to firsttransmitter coil 122 and second transmitter coil 124. The AC signal maybe, but is not limited to, a square wave, a sinusoidal wave, atriangular wave, or a sawtooth wave. In one embodiment, power circuit126 includes a resonant capacitor. Controller 146 sends one or morecontrol signals to power circuit 128 to control when the AC signal isapplied to the transmitter coil structure and to control the parameters(e.g., magnitude and frequency) of the AC signal. When a current 152flows through first transmitter coil 122 in a counter-clockwise spatialdirection as shown in FIG. 1, the counter-clockwise flow of current 152through first transmitter coil 122 generates a magnetic field. Accordingto the “right-hand-rule,” the counter-clockwise flow of current 152through first transmitter coil 122 causes flux lines (not shown) of themagnetic field to flow in an upward spatial direction. When a current154 flows through second transmitter coil 124 in a clockwise spatialdirection as shown in FIG. 1, the clockwise flow of current 154 throughsecond transmitter coil 124 generates a magnetic field. According to the“right-hand-rule,” the clockwise flow of current 154 through secondtransmitter coil 124 causes flux lines (not shown) of the magnetic fieldto flow in a downward spatial direction.

In the FIG. 1 embodiment, first transmitter coil 122 is coupled inseries with second transmitter coil 124 in such a way that at a givenpoint in time current 152 is equivalent in magnitude to current 154 butflows in an opposite spatial direction. If first transmitter coil 122and second transmitter coil 124 are substantially identical, the flow ofcurrent 152 through first transmitter coil 122 generates a magneticfield substantially equivalent in magnitude to the magnetic fieldgenerated by the flow of current 154 through second transmitter coil124. Because current 152 and current 154 are flowing in opposite spatialdirections at any given point in time, the magnetic field generated bycurrent 152 has a different polarity than the magnetic field generatedby current 154. Although in the embodiment of FIG. 1 first transmittercoil 122 and second transmitter coil 124 are coupled together in series,in other embodiments first transmitter coil 122 and second transmittercoil 124 are coupled together in parallel to achieve the same result ofmagnetic fields with opposite polarities. Embodiments of an oppositepolarity coil transmitter are further described in U.S. patentapplication Ser. No. 15/082,533, entitled “Wireless Power Transfer UsingMultiple Coil Arrays,” the subject matter of which is herebyincorporated by reference in its entirety.

First sensor coil 132 shares a common central axis with firsttransmitter coil 122, and second sensor coil 134 shares a common centralaxis with second transmitter coil 124. In one embodiment, first sensorcoil 132 and second sensor coil 134 have the same number of turns,substantially the same area, and are wound in the same direction. Themagnetic field generated by first transmitter coil 122 induces a firstsensor voltage in first sensor coil 132, and the magnetic fieldgenerated by second transmitter coil 124 induces a second sensor voltagein second sensor coil 134. For example, as shown in FIG. 1, when current152 is flowing in first transmitter coil 122, by Faraday's law ofinduction, a voltage is induced in first sensor coil 132 having apolarity that generates a magnetic field that attempts to oppose theflux generated by first transmitter coil 122, so an induced current 162a would flow in a clockwise spatial direction in first sensor coil 132.Similarly, when current 154 is flowing in second transmitter coil 124,by Faraday's law of induction, a voltage is induced in second sensorcoil 134 having a polarity that generates a magnetic field that attemptsto oppose the flux generated by second transmitter coil 124, so aninduced current 164 a would flow in a counter-clockwise spatialdirection in second sensor coil 134. First sensor coil 132 is coupled tosecond sensor coil 134 by a pathway 136 such that a first inducedcurrent 162 b (substantially the same as induced current 162 a) flows ina direction opposite to a second induced current 164 b (substantiallythe same as induced current 164 a). Typically, under steady conditions,with no wireless power receiver present or with one or more wirelesspower receiver coils (such as the longitudinal receiver coils disclosedin U.S. patent application Ser. No. 15/448,196, the subject matter ofwhich is hereby incorporated by reference in its entirety) are locatedsymmetrically between first transmitter coil 122 and second transmittercoil 124, the magnitudes of first induced current 162 b and secondinduced current 164 b are substantially equal. As first induced current162 b and second induced current 164 b are flowing in oppositedirections and are substantially equal in magnitude, no net currentflows in the foreign object sensor coil structure.

Voltage detector 142 is coupled to first sensor coil 132 and secondsensor coil 134, and detects a net sensor voltage (i.e., the sum of thefirst sensor voltage and the second sensor voltage) across the foreignobject sensor coil structure. Voltage detector 142 includes a senseresistor 144 that is coupled between first sensor coil 132 and secondsensor coil 134. In the case where no net current is flowing in firstsensor coil 132 and second sensor coil 134, no voltage develops acrosssense resistor 144. In a case where a net current is flowing in firstsensor coil 132 and second sensor coil 134, for example when a foreignmetal object is present on a transmitting surface (not shown in FIG. 1)of transmitter 100, a voltage develops across sense resistor 144.Voltage detector 142 rectifies and filters the voltage across senseresistor 144 to generate a DC signal that represents the magnitude ofthe net sensor voltage. Voltage detector 142 provides the signal thatrepresents the magnitude of the net sensor voltage to controller 146. Inone embodiment, controller 146 is a microcontroller; in otherembodiments controller 146 is a microprocessor, an application specificintegrated circuit (ASIC), or a field programmable gate array (FPGA).Controller 146 evaluates the magnitude of the net sensor voltage todetermine whether a metallic foreign object such as a coin is present onthe transmitting surface. In one embodiment, controller 146 compares themagnitude of the net sensor voltage to a predetermined threshold and ifthe magnitude of the net sensor voltage is greater than thepredetermined threshold, controller 146 determines that a foreign objectis present on the transmitting surface. If the magnitude of the netsensor voltage is not greater than the predetermined threshold,controller 146 determines that no foreign object is present on thetransmitting surface. When controller 146 has determined that a foreignobject is present on the transmitting surface, in one embodimentcontroller 146 sends a control signal to power circuit 128 to haltproviding power to first transmitter coil 122 and second transmittercoil 124. In another embodiment, controller 146 sends an alarm signal toan output device (not shown) that generates a user-perceivable output(e.g., a sound, a flashing light, or a combination of audio, visual,and/or tactile output) to signal that a foreign object is present on thetransmitting surface.

FIG. 2 is a cross-sectional diagram of one embodiment of a wirelesstransmitter coil structure and a foreign object sensor coil structure,according to the invention. First transmitter coil 122 and secondtransmitter coil 124 of the transmitter coil structure and first sensorcoil 132 and second sensor coil 134 of the foreign object sensor coilstructure underlie a transmitting surface 212. In one embodiment,transmitting surface 212 is made of a non-conducting material such asglass or plastic and is able to support at least one portable electronicdevice, for example a smartphone, tablet, or laptop. The pair of coilsconsisting of first transmitter coil 122 and first sensor coil 132 andthe pair of coils consisting of second transmitter coil 124 and secondsensor coil 134 are arranged symmetrically about an axis 240.

Current 152 flowing in first transmitter coil 122 generates a magneticfield represented by flux lines 232, which flow in an upward direction.Current 154 flowing in second transmitter coil 124 generates a magneticfield represented by flux lines 234, which flow in a downward direction.Because flux lines 232 and flux lines 234 flow in opposite directions,the magnetic reluctance between flux lines 232 and flux lines 234 islow, causing flux lines 232 and flux lines 234 between first coil 122and second coil 124 to attract to each other. Flux lines 232 and fluxlines 234 magnetically couple between first transmitter coil 122 andsecond transmitter coil 124 to form closed flux lines 236.

The magnetic field generated by first transmitter coil 122 induces afirst sensor voltage in first sensor coil 132 and the magnetic fieldgenerated by second transmitter coil 124 induces a second sensor voltagein second sensor coil 134. First sensor coil 132 and second sensor coil134 are coupled together in such a way (as shown in FIG. 1) that thefirst sensor voltage has a first polarity and the second sensor voltagehas a second polarity that is opposite to the first polarity. When noforeign object is present on transmitting surface 212, the magnitude ofthe first sensor voltage of first sensor coil 132 is substantially equalto the magnitude of the second sensor voltage of second sensor coil 134because each of first sensor coil 132 and second sensor coil 134receives substantially the same amount of energy from the magneticfields produced by first transmitter coil 122 and second transmittercoil 124. In most embodiments, the magnitudes of the first sensorvoltage and the second sensor voltage will not be exactly equal when noforeign object is present due at least in part to slight differencesbetween first sensor coil 132 and second sensor coil 134 (e.g., slightlydifferent resistances or other manufacturing tolerances), any asymmetrybetween first sensor coil 132 and second sensor coil 134 with respect toaxis 240, any asymmetry between the central axis of first sensor coil132 and the central axis of first transmitter coil 122, and/or anyasymmetry between the central axis of second sensor coil 134 and thecentral axis of second transmitter coil 124. The first sensor voltageand the second sensor voltage also may not be equal when no foreignobject is present if a receiver coil of a receiving device is notlocated symmetrically with respect to axis 240. In one embodiment, aforeign object detection threshold is predetermined by detecting themagnitude of the net voltage across first sensor coil 132 and secondsensor coil 134 (i.e., the net voltage across the foreign object sensorcoil structure) when first transmitter coil 122 and second transmittercoil 124 are generating a magnetic field and it is known that no foreignobject is present on transmitting surface 212.

A coin 220 located on transmitting surface 212 as shown in FIG. 2receives a portion of the energy of the magnetic fields, causing eddycurrents to flow in coin 220. The presence of coin 220 on transmittingsurface 212 reduces the amount of energy of the magnetic field availableto first sensor coil 132, with the result that the first sensor voltageinduced in first sensor coil 132 will be less than it would otherwise bewhen coin 220 is not present. When coin 220 is present, the first sensorvoltage induced in first sensor coil 132 will not be substantially equalto the second sensor voltage induced in second sensor coil 134. Voltagedetector 142 (not shown in FIG. 2) detects a net voltage across firstsensor coil 132 and second sensor coil 134 (i.e., the foreign objectsensor coil structure). When a foreign object such as coin 220 is ontransmitting surface 212, the magnitude of the net voltage across theforeign object sensor coil structure will be greater than thepredetermined threshold that represents the net voltage magnitude whenno foreign object is present.

FIG. 3 is a diagram of one embodiment of a foreign object detectionsystem, according to the invention. The foreign object detection systemof FIG. 3 is a sub-system of transmitter 100 of FIG. 1. A magnetic fieldrepresented by flux lines 232 and the upward flowing portions of fluxlines 236 generated by first transmitter coil 122 (not shown in FIG. 3for clarity) induces a first sensor voltage (V1) 312 in first sensorcoil 132. First sensor voltage 312 causes induced current 162 a to flowin first sensor coil 132. A magnetic field represented by flux lines 234and the downward flowing portions of flux lines 236 generated by secondtransmitter coil 124 (not shown in FIG. 3 for clarity) induces a secondsensor voltage (V2) 314 in second sensor coil 134. Second sensor voltage314 causes induced current 164 a to flow in second sensor coil 134. Asshown in FIG. 3, first sensor coil 132 is coupled to second sensor coil134 in such a way that first sensor voltage 312 and second sensorvoltage 314 have opposite polarities at any given point in time. Voltagedetector 142 detects a net voltage (Vnet) 316 that is the sum of firstsensor voltage 312 and second sensor voltage 314, as first sensor coil132 and second sensor coil 134 are coupled together in series. In oneembodiment, voltage detector 142 presents a high impedance to theforeign object sensor coil structure so currents flowing in first sensorcoil 132 and second sensor coil 134 will be very small (on the order ofmicroamps). Voltage detector 142 rectifies and filters the voltageacross sense resistor 144 to generate a DC signal that represents themagnitude of the net sensor voltage. In one embodiment, voltage detector142 includes a diode bridge circuit to rectify the voltage across senseresistor 144 and a filter circuit to generate the DC signal appropriateto be input to controller 146.

When no metallic foreign object is present on a transmitting surface(not shown), first sensor coil 132 and second sensor coil 134 receivesubstantially the same amount of energy from the magnetic fieldsgenerated by the transmitter coil structure and the magnitude of firstsensor voltage 312 and the magnitude of second sensor voltage 314 areapproximately equal. Because at any given point in time first sensorvoltage 312 and second sensor voltage 314 have opposite polarities, netvoltage 316 will be a small value (e.g., close to zero) when no foreignobject is present on the transmitting surface. At set forth above, inone embodiment, a threshold is predetermined by detecting the magnitudeof net voltage 316 when first transmitter coil 122 and secondtransmitter coil 124 are generating a magnetic field and it is knownthat no foreign object is present on the transmitting surface. Thisthreshold accounts for manufacturing tolerances of transmitter 100.

When a metallic foreign object such as a coin or key is present on thetransmitting surface while the transmitter coils 122 and 124 aregenerating magnetic fields, the eddy currents that develop in theforeign object will reduce the amount of energy that one of first sensorcoil 132 and second sensor coil 134 will receive from the magneticfields. In such a case, the magnitudes of first sensor voltage 312 andsecond sensor voltage 314 will no longer be substantially equal and themagnitude of net voltage 316 will be greater than the predeterminedthreshold. Voltage detector 136 detects net voltage 316 and sends asignal representing the magnitude of net voltage 316 to controller 140.Controller 140 compares the magnitude of net voltage 316 to thepredetermined threshold, and if the magnitude of net voltage 316 isgreater than the predetermined threshold, controller 140 determines thata foreign object is present on the transmitting surface. In oneembodiment, upon determining that a foreign object is present on thetransmitting surface, controller 140 instructs power circuit 126 to haltproviding power to the transmitter coil structure. In one embodiment,controller 140 also sends an alarm signal to an output device (notshown), which in response to the alarm signal generates auser-detectable output, for example a sound or a flashing light, toindicate that a foreign object is present on the transmitting surface.

FIG. 4 is a flowchart of method steps for detecting a foreign object ona transmitting surface of a wireless power transmitter, according to oneembodiment of the invention. In a step 410, controller 146 sends acontrol signal to power circuit 128 to apply less than full power to thetransmitter coil structure (i.e., first transmitter coil 122 and secondtransmitter coil 124). In one embodiment, controller 146 causes powercircuit 128 to apply a voltage with a predetermined magnitude that isless than the magnitude appropriate for wireless power transfer. In oneembodiment, controller 146 causes power circuit 128 to apply a voltagewith a gradually increasing magnitude up to a predetermined maximumvalue. Less than full power is initially applied to the transmitter coilstructure in the FIG. 4 embodiment to enable detection of apossibly-present foreign object without the foreign object developing apotentially dangerous temperature. In a step 412, voltage detector 142detects a net voltage across the foreign object sensor coil structure(i.e., first sensor coil 132 and second sensor coil 134). Voltagedetector 142 then sends a signal that represents the magnitude of thenet voltage to controller 146. In a step 414, controller 146 comparesthe magnitude of the net voltage to a predetermined threshold. If themagnitude of the net voltage is not greater than the predeterminedthreshold, then in a step 416 controller 146 determines that no foreignobject is present and causes power circuit 128 to apply full power tofirst transmitter coil 122 and second transmitter coil 124. If themagnitude of the net voltage is greater than the predeterminedthreshold, then in a step 418 controller 146 determines that a foreignobject is present and sends an alarm signal to an output device to causethe output device to generate a user-detectable alarm such as a sound ora flashing light. The method then returns to step 412 where voltagedetector 142 continues to detect the net voltage across first sensorcoil 132 and second sensor coil 134. In one embodiment, steps 412, 414,and 418 are repeated until the magnitude of the net voltage is less thanthe predetermined threshold (for instance if the foreign object isremoved from the transmitting surface) or until a predetermined amountof time has expired. If the predetermined amount of time has expired,controller 146 then causes power circuit 128 to halt providing power tothe transmitter coil structure.

FIG. 5 is a flowchart of method steps for detecting a foreign object ona transmitting surface of a wireless power transmitter, according to oneembodiment of the invention. In a step 510, controller 146 sends acontrol signal to power circuit 128 to apply less than full power to thetransmitter coil structure (i.e., first transmitter coil 122 and secondtransmitter coil 124). In one embodiment, controller 146 causes powercircuit 128 to apply a voltage with a predetermined magnitude that isless than the magnitude appropriate for wireless power transfer. In oneembodiment, controller 146 causes power circuit 128 to apply a voltagewith a gradually increasing magnitude up to a predetermined maximumvalue. In a step 512, voltage detector 142 detects a net voltage acrossthe foreign object sensor coil structure (i.e., first sensor coil 132and second sensor coil 134). Voltage detector 142 then sends a signalthat represents the magnitude of the net voltage to controller 146. In astep 514, controller 146 compares the magnitude of the net voltage to apredetermined threshold. If the magnitude of the net voltage is notgreater than the predetermined threshold, then in a step 516 controller146 causes power circuit 128 to apply full power to first transmittercoil 122 and second transmitter coil 124. If the magnitude of the netvoltage is greater than the predetermined threshold, then in a step 518controller 146 causes power circuit 128 to halt providing power to thetransmitter coil structure.

The invention has been described above with reference to specificembodiments. It will, however, be evident that various modifications andchanges may be made thereto without departing from the broader spiritand scope of the invention as set forth in the appended claims. Theforegoing description and drawings are, accordingly, to be regarded inan illustrative rather than a restrictive sense.

What is claimed is:
 1. A wireless power transfer system comprising: atransmitter coil structure comprising a first transmitter coil, and asecond transmitter coil coupled to the first transmitter coil in such away that when a first current flows in the first transmitter coil in afirst spatial direction the first current flows in the secondtransmitter coil in a second spatial direction different from the firstspatial direction; a foreign object sensor coil structure comprising afirst sensor coil having a central axis in common with the firsttransmitter coil, and a second sensor coil coupled in series to thefirst sensor coil, the second sensor coil having a central axis incommon with the second transmitter coil, the first sensor coil coupledto the second sensor coil in such a way that when a first voltageinduced in the first sensor coil has a first polarity a second voltageinduced in the second sensor coil has a second polarity different fromthe first polarity; a voltage detector coupled to the foreign objectsensor coil structure, the voltage detector configured to detect a netvoltage across the foreign object sensor coil structure; and acontroller configured to receive the net voltage and determine apresence or absence of a foreign object based on a magnitude of the netvoltage.
 2. The wireless power transfer system of claim 1, wherein thecontroller is configured to determine the presence of the foreign objectwhen the magnitude of the net voltage is greater than a predeterminedthreshold and determine the absence of the foreign object when themagnitude of the net voltage is not greater than the predeterminedthreshold.
 3. The wireless power transfer system of claim 1, furthercomprising a power circuit coupled to the transmitter coil structure,the power circuit configured to provide power to the transmitter coilstructure.
 4. The wireless power transfer system of claim 3, wherein thecontroller is further configured to cause the power circuit to haltproviding power to the transmitter coil structure upon determining thepresence of the foreign object.
 5. The wireless power transfer system ofclaim 1, further comprising an output device, and wherein the controlleris further configured to cause the output device to generate auser-perceivable output upon determining the presence of the foreignobject.
 6. The wireless power transfer system of claim 1, furthercomprising a transmitting surface overlying the transmitter coilstructure and the foreign object sensor coil structure.
 7. The wirelesspower transfer system of claim 1, wherein the voltage detector comprisesa sense resistor coupled to the foreign object sensor coil structure. 8.The wireless power transfer system of claim 1, further comprising alayer of magnetic material underlying the transmitter coil structure andthe foreign object sensor coil structure.
 9. A method for detectingforeign objects on a wireless power transfer device comprising:providing power to a transmitter coil structure comprising a firsttransmitter coil, and a second transmitter coil coupled to the firsttransmitter coil in such a way that when a first current flows in thefirst transmitter coil in a first spatial direction the first currentflows in the second transmitter coil in a second spatial directiondifferent from the first spatial direction; detecting a net voltageacross a foreign object sensor coil structure, the foreign object sensorcoil structure comprising a first sensor coil having a central axis incommon with the first transmitter coil, and a second sensor coil coupledin series to the first sensor coil, the second sensor coil having acentral axis in common with the second transmitter coil, the firstsensor coil coupled to the second sensor coil in such a way that when afirst voltage induced in the first sensor coil has a first polarity asecond voltage induced in the second sensor coil has a second polaritydifferent from the first polarity; comparing the net voltage to apredetermined threshold; and determining presence or absence of aforeign object based on a result of comparing the net voltage to thepredetermined threshold.
 10. The method of claim 9, wherein determiningpresence of the foreign object comprises determining that the netvoltage is greater than the predetermined threshold.
 11. The method ofclaim 9, wherein determining absence of the foreign object comprisesdetermining that the net voltage is not greater than the predeterminedthreshold.
 12. The method of claim 9, further comprising haltingproviding power to the transmitter coil structure in response todetermining the presence of the foreign object.
 13. The method of claim9, further comprising generating a user-perceivable output in responseto determining the presence of the foreign object.
 14. A system fordetecting foreign objects on a wireless power transmitter, comprising: aforeign object sensor coil structure comprising a first sensor coil, anda second sensor coil coupled in series to the first sensor coil, thefirst sensor coil coupled to the second sensor coil in such a way thatwhen a first voltage induced in the first sensor coil has a firstpolarity a second voltage induced in the second sensor coil has a secondpolarity different from the first polarity; a voltage detector coupledto the foreign object sensor coil structure, the voltage detectorconfigured to detect a net voltage across the foreign object sensor coilstructure; and a controller configured to receive the net voltage anddetermine a presence or absence of a foreign object based on a magnitudeof the net voltage.
 15. The system of claim 14, wherein the controlleris configured to determine the presence of the foreign object when themagnitude of the net voltage is greater than a predetermined thresholdand determine the absence of the foreign object when the magnitude ofthe net voltage is not greater than the predetermined threshold.
 16. Thesystem of claim 14, further comprising an output device, and wherein thecontroller is further configured to cause the output device to generatea user-perceivable output upon determining the presence of the foreignobject.
 17. The system of claim 14, further comprising a transmittingsurface overlying the foreign object sensor coil structure.
 18. Thesystem of claim 14, wherein the voltage detector comprises a senseresistor coupled to the foreign object sensor coil structure.
 19. Thesystem of claim 14, further comprising a layer of magnetic materialunderlying the foreign object sensor coil structure.